For example, a user searching for "baseball glove" might click on a baseball news blog in the search results. The words "baseball glove" used in the original query would appear in the referral URL passed on to the blog's site.

AdSense treats the query words in the referral URL as part of the content on the Web page and uses the overall content to figure out which are the most relevant ads. It doesn't matter where the search originated -- the query could have come from Microsoft's (MSFT, Fortune 500) Bing, Yahoo! (YHOO, Fortune 500) or another search engine.

Now, Google says it has "recently started to expand the use of the query words ... to a few hours so we can continue to deliver more relevant ads."

After "a few hours," Google wrote, the query words are no longer treated as part of the sites' content to help deliver ads. And if a user starts a new search, the process begins all over again -- AdSense does not add new query words to the referral URL.

The referral URL is one of several pieces of information that Google uses to target ads. Users can opt out of the advertising "cookie," or tracking file, used for AdSense. If users opt out they will receive less targeted ads.

Karsten Weide, media analyst at IDC said consumers don't need to be concerned, as Google doesn't know anything about the user behind the machine.

"It only identifies your computer -- they don't know your name or address, and frankly, they don't care," Weide said. "All they need to know is this person at this machine for a few hours has this certain interest at the moment."

Google's move will likely benefit advertisers more in the short term, but over time consumers may change how they view ad targeting, Weide said.

"In 20 years, we'll see this targeted content fit seamlessly with what we're interested in," Weide said. "It will be so well-matched that we won't even think of it as marketing."

Correction: A previous version of this article incorrectly stated that AdSense is an ad server, and that it uses search history to target ads. It also incorrectly stated that the changes described in this article also affect ads seen on Google.com searches.

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